Prior to this invention it has been proposed to electrically detect touchdown of a tennis ball in one or more selected areas of a tennis court by arranging a plurality of exposed spaced apart electrical conductors of a sensing circuit in the selected area or areas and by providing the ball with an electrically conductive means which is capable of bridging and, hence, completing a metallic circuit between two or more of the sensing circuit's conductors upon touchdown of the ball in the selected area. The conductors in the sensing circuit are connected to a power source in such a manner that contact of the ball with any pair of adjacent conductors in the sensing circuit completes a current-conducting path for signalling touchdown of the ball in the selected area.
Various tennis balls specifically constructed for this purpose have been proposed as described, for example, in U.S. Pat. No. 3,854,719 issued on Dec. 17, 1974 to L. D. Supran, U.S. Pat. No. 3,883,860 issued on May 13, 1975 to H. Von Kohorn, and Swedish patent No. 206,864.
In one type of prior tennis ball construction, the electrically conductive means comprises steel or other metal conductors woven into the cloth cover panels so that the cover of the ball is a composite of woven metal and fabric strands or fibers. In another type of prior tennis ball construction, the electrically conductive means takes the form of an electrically conductive coating or paint which is applied to the cover.
The latter of the two types of electrically conductive tennis ball constructions mentioned above is not acceptable for normal usage because the electrically conductive coating will wear off prematurely to objectionably shorten the effective life of the ball. On the other hand, previously disclosed tennis balls which employ a sufficient quantity of steel or other electrically conductive metal fibers or elements to assure completion of a circuit between conductors in the electrical sensing circuit are much heavier than standard non-conductive balls and are not known to meet official weight and other official requirements or standards.
Additionally, previously disclosed tennis balls employing steel or other metal coated fibers, strands or elements have the effect of objectionably discoloring tennis ball covers.
These features of the prior electrically conductive type balls have inhibited acceptance of electrical detection systems of the type described above in tennis and other games employing a ball.
U.S. Pat. No. 3,774,194 (issued to P. Jokay et al on Nov. 20, 1973) also discloses a special tennis ball constuction for use with an electrical detection system, but unlike the balls described in the three patents mentioned above, the ball in Jokay et al employs a resonant circuit which affects a radio frequency signal transmitted to a receiving antenna in a receiver.